Answer: -I would suggest you start your search for information with whatever body is responsible for building codes where you live (any competent building contractor should be able to steer you) and find out first if there is a legal requirement for one size or the other in new home plumbing. That might make the upgrade question a moot point. -The main supply lines should be 3/4" and the branch lines to the fixture shut-offs should be 1/2". When talking about a $120k+ new home, IMO the incremental cost for the larger copper pipe (all 30 ft ? of it and fittings) is negligible. As to the difference in flow that it will make, do the arithmetic comparing the cross-sectional areas.